Conventionally, an optical modulation technology that is the mainstream in optical communication systems these days has been on-off key (OOK, that is, binary shift keying) in return-to-zero (RZ) or non-return-to-zero (NRZ) format. In the case of OOK, the transmission rate of an optical signal is 10 Gbps.
An optical signal output from an optical communication device on the transmitting side is propagated by an optical transmission network, which is a wavelength division multiplexing (WDM) channel. The optical transmission network includes an optical wavelength division multiplexer, an optical amplifier, and a wavelength division demultiplexer in its line.
When the optical amplifier amplifies an optical signal, the S/N ratio of the optical signal deteriorates. Furthermore, in optical transmission over a long distance, wavelength dispersion occurs because of nonlinear characteristics of fiber cables, resulting in waveform distortion. The waveform distortion of the optical signal caused by the wavelength dispersion increases approximately in proportion to the square of the transmission rate.
In recent years, along with large transmission capacity and high-speed optical communications asked for such an optical transmission system, a system for reducing the influence of the wavelength dispersion needs to be developed. The optical modulation technologies in which the transmission rate of an optical signal is 40 Gbps are developed actively and are commercialized. Specifically, the optical modulation technologies such as a duo binary system, a carrier-suppressed return-to-zero (CSRZ) system, a differential phase shift keying (DPSK) system, a binary phase shift keying (BPSK) system, and a differential quadrature phase shift keying (DQPSK) system are used.
As described above, because the waveform distortion of an optical signal caused by the wavelength dispersion increases in proportion to the square of the transmission rate of the optical signal, compensation of the wavelength dispersion becomes more important as the transmission rate of the optical signal is increased. The compensation of the wavelength dispersion in the optical network, which is the WDM channel, includes a method for performing dispersion compensation on each wavelength and a method for performing dispersion compensation on all wavelengths collectively.
The method for performing dispersion compensation on each wavelength requires higher costs than the method for performing dispersion compensation on all wavelengths collectively. By contrast, in the collective dispersion compensation method, wavelength dispersion slopes indicating a slope of dispersion at the zero point of the wavelength are different depending on transmission lines of the optical transmission network. As a result, transmission line dispersion cannot be dispersed and compensated perfectly for all of the wavelengths.
Therefore, in recent years, in the optical communication system whose transmission rate is 40 Gbps, using a variable dispersion compensator for each channel in a direct detection method, such as DPSK and DQPSK, to compensate wavelength dispersion of an optical signal has become the mainstream.
For example, International Publication Pamphlet No. WO 1999/048231 discloses a conventional technology in which wavelength dispersion caused by a transmission line of an optical transmission network is perturbed, errors generated when the former and the latter phases are perturbed are counted and compared, and the center of the perturbation is updated in the direction with fewer errors to compensate the dispersion.
Furthermore, Japanese Laid-open Patent Publication No. 2005-286382 discloses another conventional technology in which dispersion compensation is performed without perturbing wavelength dispersion in a variable dispersion compensator by using characteristics of an optical signal in that the symbol error number or the symbol error rate of each symbol changes at a certain fixed discrimination point depending on a direction of positive dispersion or negative dispersion in residual dispersion caused by temperature change or other factors.
However, in one of the conventional technologies described above, in order to perturb the wavelength dispersion, distortion needs to be generated intentionally in the received demodulated waveform itself. As a result, in addition to deterioration in the Q value, for example, the quality of an extracted clock deteriorates. Furthermore, unexpected malfunction may be induced. For example, the perturbation induces an increase in intersymbol interference, and frequency components included in a band of transmission characteristics of a clock extractor among the frequency components of the distorted waveform are transmitted without any change, thereby increasing jitter of the clock.
Furthermore, in the other of the conventional technologies described above, even if the residual dispersion occurs, when the symbol error numbers of the symbols of the optical signal at the certain fixed discrimination point are approximately equal to each other, it is difficult to detect the residual dispersion from symbol error information of the symbols.